P1260: UNRAVELING THE GENETICS OF TRANSFORMED SPLENIC MARGINAL ZONE LYMPHOMA
Authors: M. Grau, C. López, A. Navarro, G. Clot, F. Nadeu, G. Bastidas, M. Alcoceba, M. J. Baptista, M. Blanes, F. Climent, D. Colomer, D. Costa, E. Domingo-Domènech, P. Forcada, A. Enjuanes, L. Escoda, G. Frigola, E. Giné, M. Lopez-Guerra, A. Rivas-Delgado, L. Vicente-Folch, A. Wotherspoon, E. Campo, A. López-Guillermo, E. Matutes, S. Beà
Journal: HemaSphere (2022)

Background: Splenic marginal zone lymphoma (SMZL) is an indolent non-Hodgkin lymphoma but about 10-15% of patients may undergo transformation to an aggressive lymphoma, commonly diffuse large B-cell lymphoma (DLBCL). Although SMZL genomic profile has been largely investigated, the genetic mechanisms leading to this transformation are still unknown. Aims: To characterize the genomic landscape of transformed SMZL in order to establish the underlying mechanisms associated with transformation and the potential clonal evolution during this event. Methods: We investigated 42 SMZL that underwent DLBCL transformation. Tumor material was obtained at diagnosis and transformation in 19 patients, only at transformation in 14 patients, and only at diagnosis in 9 (61 total samples). The pathology and flow cytometry data of all cases were reviewed. Samples were categorized in two groups: i) at diagnosis (SMZL, n=28 samples), and ii) at transformation (SMZL-T, n=33 samples). DNA was extracted from formalin-fixed paraffin-embedded (FFPE) tissue (n=45), frozen tissue biopsies (n=10), or PB (n=6). Single nucleotide variants (SNV) and indels were analyzed in all samples (n=61) using a SMZL-panel capturing 37 genes and custom bioinformatic pipelines. Copy number alterations (CNA) were assessed using OncoScan TM CNV FFPE (n=48) or CytoScan HD (n=3) arrays, and analyzed using Nexus Biodiscovery software. Whole genome sequencing (WGS) was performed in one case (diagnostic and transformed tumor and germline DNA samples). The IGH clonal relationship between samples at diagnosis and transformation was examined in 17/19 cases. Results: Integrating mutations and CNA, the most frequently altered genes in the 33 SMZL-T samples were TNFAIP3 (57.6%), KMT2D (48.5%), KLF2 (33.3%), TP53 (33.3%), and ARID1A (30.3%). The most frequent CNA were gains of 1q (39.3%), 6p (28.6%) and 3q (27.3%); and losses of 9p21.3 (CDKN2A/B) (39.3%), and 7q31-q32 (33.3%). SMZL-T had higher number of CNA than SMZL at diagnosis (8 vs 5; P<0.001). Moreover, genetic alterations of TNFAIP3, 9p21.3 (CDKN2A/B), TP53 and 6p were significantly enriched at transformation as compared to diagnosis (57.6% vs 28.6%, P<0.001; 39.3% vs 10.7%, P=0.001; 33.3% vs 14.3%, P=0.04; and 28.6% vs 13%, P=0.05, respectively). All paired samples with available results (n=12) were clonally related. We evaluated the clonal evolution of SMZL/SMZL-T with complete NGS and CNA data (n=14) and identified a divergent evolution pattern in the majority of the cases (13/14, 93%), with only one case showing a linear evolution. Finally, the WGS analysis revealed the following alterations in SMZL and SMZL-T samples respectively: 62 and 76 coding SNVs/indels, 9 and 11 CNA, 14 and 19 interchromosomal translocations and insertions. About half of the aberrations (52%) were shared by diagnostic and transformation samples. Interestingly, we identified an IGH::BCL3 translocation, displaying the breakpoints at the switch region IGHG2 and downstream of BCL3 in the diagnostic and transformation samples. This case did not have any of the characteristic alterations of SMZL. Summary/Conclusion: Taken together, SMZL-T are characterized by higher genomic complexity than SMZL and have specific genomic alterations acquired during transformation including aberrations in TNFAIP3 and TP53, focal losses at 9p21.3 (CDKN2A/B) and 6p gains, suggesting that these alterations may represent key players in SMZL transformation. A divergent evolution pattern was observed in virtually all cases.